The major histocompatibility complex (MHC) plays a key role in regulating the immune response, but MHC molecules may also have a more generalized role in a variety of cellular functions. MHC molecules associate with cell surface molecules involved in cell growth and also share many physical characteristics with receptor molecules. It is thus likely that MHC molecules may have receptor functions as well. This proposal focuses on two additional functions of MHC molecules that we recently described: the regulation of the cytolytic activity of human natural killer (NK) cells, and a role in the activation and proliferation of human T and B lymphocytes and NK cells. The major goal of this project is to determine the mechanism by which MHC cell surface molecules affect both cytolytic activity and lymphocyte growth and activation. Monoclonal antibodies (mAbs) with specificity for HLA class I and class II molecules were generated which affect the lytic activity of a cloned line of human NK cells (NK 3.3). The HLA class I- specific mAb that augments lysis (mAb 131) has been characterized as a unique anti-HLA-A locus-specific mAb. In contrast, an anti-HLA-B locus-specific mAb (4E) inhibits killing, suggesting HLA class I-locus-specific effects on NK function. Similarly, the anti-HLA-A and anti-HLA-B locus-specific mAbs have differential effects on the activation of T and B lymphocytes by a variety of mitogens. Such functional differences in HLA-A and B locus-encoded gene products have not been previously described. These mAbs are thus valuable tools for understanding the role of MHC molecules in lymphocyte functions. The working hypothesis is: 1) MHC molecules associate with functionally important receptors in the membranes of NK cells, T cells and B cells, and 2) HLA-A locus and B locus gene products form different associations resulting in different functional effects of the antibodies directed against them. We will test this hypothesis by: 1) examining potential physical associations of MHC class I A and B locus-encoded molecules with other functionally important molecules on NK, T and B cells, 2) studying differential recycling and phosphorylation of MHC molecules in these cells, and 3) studying changes in early metabolic events and gene expression associated with activation or lytic activity in NK, T and B cells in the absence and presence of the anti-HLA and B locus-specific mAbs. These studies will provide further information on the role of the MHC in cellular functions, including growth, and provide greater insight into the mechanisms and regulation of lytic function and cellular proliferation.